The inactivation of retroviral vectors derived from murine cells in human serum represents a formidable barrier for many potential applications of in vivo retroviral-mediated gene therapy. We have recently demonstrated that the inactivation of amphotropic retroviral vector particles in human serum is primarily mediated by complement activation initiated through natural antibody recognition of the alpha-galactosyl epitope on the retroviral envelope. Furthermore, inactivation of retroviral particles in human serum was completely abolished when the interaction between anti- alpha-galactosyl Ab and this viral epitope was inhibited. The involvement of the alpha-galactosyl epitope in the inactivation of the murine retroviral vector makes it a good target in the development of complement resistant vectors. The aim of this proposal is to develop retroviral vector particles that are resistant to human complement-mediated inactivation. Specifically, we propose to investigate the ability of retroviral vector particles to survive in human serum when these particles are generated from producer cells in which expression of the alpha- galactosyl epitope is reduced or eliminated. Successful generation of complement resistant vectors will allow attempts at retroviral-mediated gene transfer experiments in vivo in applications where contact with the human complement system is unavoidable. PROPOSED COMMERCIAL APPLICATION: The potential of retroviral-mediated gene transfer will not be fully realized until vectors can be effectively administered in vivo. The rapid inactivation of currently used retroviral vectors in human serum represents a major obstacle for attempting such procedures. The generation of serum resistant retroviral vectors would represent a major technological breakthrough for in vivo retroviral- mediated gene transfer experiments.